Olefins such as ethylene, propylene, and butylene may be produced by heating saturated hydrocarbons such as ethane, propane, or butane at elevated temperatures. Likewise, naphtha, gas oil, and other heavy hydrocarbon feeds may be thermally cracked in a cracking furnace in the presence of steam to produce olefins.
The cracking effluent produced by heating a saturated hydrocarbon, naphtha, or gas oil feed typically contains hydrogen, steam, carbon dioxide, carbon monoxide, methane, ethane, ethylene, propane, propylene, and minor amounts of other components such as heavy hydrocarbons. The cracking effluent is then sent to a product recovery section of the olefins plant.
In the product recovery section, the cracking effluent is compressed in one or more compression stages to partially liquefy the hydrocarbon components for separation via cryogenic distillation. Carbon dioxide, steam, and heavy hydrocarbons must be removed prior to chilling the cracking effluent to prevent them from freezing and plugging the equipment. After removal of these components from the cracking effluent, the effluent is passed to a cryogenic section (commonly referred to as a "Cold Box") where the temperature of the effluent is reduced such that separation of the hydrocarbon components can be performed by distillation. The refrigeration balance of the Cold Box is provided by an ethylene refrigeration cycle for the warmer part of the Cold Box and by expanders of off-gas streams for the colder part of the Cold Box.
The distillation section typically contains three columns, a demethanizer which removes the light ends, a deethanizer which removes the heavy ends, and an ethane/ethylene splitter which separates the ethylene product from the ethane recycle stream. The reboil and condensing duties of the distillation section are also provided by the ethylene refrigeration cycle.
Hydrogen contained in the cracked gases is used, in part, for balancing the cold end of the cryogenic section. However, its presence requires colder temperatures in the distillation section to separate the products. Hydrogen also acts as a ballast in the distillation section, which prevents additional quantities of products from being processed.
In view of the drawbacks associated with the presence of hydrogen in the cracking effluent, various methods have been proposed to remove hydrogen from the cracking effluent. See, e.g., U.S. Pat. Nos. 5,082,481, 5,452,581, and 5,634,354; the contents of which are hereby incorporated by reference. The methods described in these patents include the use of a membrane separator to remove hydrogen from the cracking effluent.
However, there are several drawbacks associated with these methods. For example, unless the disclosed methods employ very selective membranes, varying amounts of products are lost in the permeate stream. Even when using highly selective membranes, the hydrogen rejection rate may not be sufficiently high to make the process commercially viable.
Accordingly, there is a need in the art for a process that minmizes or eliminates product losses in the permeate stream without the need to use very selective membranes. In addition, there is a need in the art for a process that can employ higher hydrogen rejection rates without the concomitant loss of product.